Automatic control of hot-blast temperatures



Aug. 22, 1950 "r. H. KENNEDY ETAL 2,519,475

AUTOMATIC CONTROL OF HOT BLAST TEHPERATURES Filed Oct. 21, 1944 4 Sheets-Sheet 1 INVENTORS 77PUMANH Kf/V/VED V 5/70 EMMo/vs ZMORTSON,

Aug. 22, 1950 T. H. KENNEDY ETAL AUTOMATIC CONTROL OF HOT BLAST TEMPERATURES Filed Oct. 21, 1944 4 Sheets-Sheet 2 INVENTORS I'll-Fl lll'lll lllllllr Ill-III Aug. 22, 1950 T. H. KENNEDY EI'AL 2,519,475

AUTOMATIC CONTROL OF HOT BLAST TEMPERATURES Filed Oct. 21, 1 944 4 Sheets-Sheet 3 V 11; a l

V :ZIIIIIIIIIII; I

INVENTORS 1950 -r. H. KENNEDY ETAL 2,519,475

AUTOMATIC CONTROL m HOT' BLAST TEMPERATURES Filed Oct. 21, 1944 4 Sheets-Sheet 4 I INVENTORS TRUMAN/i kw/vsoyand Patented Aug. 22, 1950 AUTOMATIC OONTROL F EDT-BLAST TEMPERATURES Truman 11. Kennedy, McKeesport, and Emmons 1'. Mortson, Pittsburgh, Pa., assignors to National Tube Company, a corporation of New Jersey Application October 21, 1944, Serial No. 559,856

2 Claims. 1

This invention relates to the control of hot blast temperatures whereby a metallurgical blast furnace is maintained at its optimum operating condition.

Among the objects of the invention is the provision of apparatus for automatically varying the temperature of the hot blast supplied to the furnace in accordance with the absolute humidity of the unheated indrawn blast air, whereby compensation for variations in absolute humidity of such air are effected by raising or lowering the temperature of the blast.

Another object is the provision of a method of regulating the temperature of the hot blast whereby the furnace operates at its optimum condition regardless of changes in the humidity of the atmosphere.

These and other objects of the invention will further appear in the following description.

It is desirable, in the interests of uniform blast furnace operation, and consequently uniformity oi product, to operate the furnace as nearly as possible under constant conditions, that is, with a uniform weight, distribution, and composition of burden, and with a uniform air blast pressure and temperature. Attainment of the last named condition has been diflicult, if not impossible, because variations in the amount of moisture in the air drawn into and heated by the blast furnace stoves results in varying markedly the effective heat content of the blast reaching the furnace, due to the fluctuating chilling effect of moisture in the indrawn air.

Our invention provides means whereby the temperatures of the blast delivered to the blast furnace is automatically varied to compensate for fluctuations in humidity of the atmosphere, whereby the chest of the blast upon the burden is made at least substantially constant regardless of such humidity variations. Our apparatus consists broadly of means which by itself would function to keep the temperature of the blast at a constant set point, plus means responsive to changes in the absolute humidity of the unheated indrawn air to vary such set point of the first means in accordance with a previously determined ratio,

degrees change from standard, blast temperature grains moisture/ft. change from standard, unheated indrawn air which is found to maintain the heating effect of the air delivered to the blast furnace at least substantially constant.

2 by reference to the accompanying drawings, in which:

Figure 1 is a schematic view of the stove and its air supply and air delivery means;

Figures 2A and 2B connected and taken as a single figure forms a partly schematic view of the means for determining the humidity of the air entering the stove;

Figure 3 is a view of the control circuit employed for varying the position of the mixer valve in accordance with the humidity of the air as determined by the apparatus of Figure 2; and

Figures 4 and 5 are views in perspective of mechanical relay mechanisms employed in the circuit of Figure 3.

As shown in Figure 1, a stove l for heating the air delivered to a metallurgical blast furnace (not shown) is supplied by main 2 with air drawn by the blowing engines from the atmosphere, said air being led into stove 9 through pipe 3 and, after being heated, being led out by pipe 4. The latter pipe feeds into main 5, which leads to the tuyeres of a blast furnace, the heated air mixing with unheated, indrawn blast air from bleed line 6, and the amount of such unheated air mixed being determined by the position of mixer valve 1, which is controlled by shaft 8.

It is obvious that the temperature of the blast air in main 5 may be varied by varying the setting of mixer valve '1. Opening of the valve I wider allows the entry of more cold air and thus reduces the temperature of the blast, whereas moving of the valve toward the closed position restricts the entry of cold air and thus raises the temperature of the blast. As stated above, the apparatus of the present invention comprises means which by itself would automatically position valve 1 to maintain the temperature of the blast air constant regardless of variations in the humidity of the atmosphere, and means for varying the constant or set temperature, at which the first control means would maintain the air blast, in accordance with variations in the humidity of the indrawn, unheated air.

The first of the above control means; namely that which by itself would maintain the temperature of the air blast constant regardless of humidity changes, consists of a thermocouple 9 positioned in air blast main 5 to determine the temperature of the heated air blast as it enters the furnace. Thermocouple 9 has leads l0 and H which are connected as shown to a control network N, shown in Figure 3. For the purpose of the present discussion, it will be assumed for The invention will be more clearly understood to the moment that there is no connection between manually set point 90.

network N and network N, means generically shown as I204: is not connected to contact or set point 90 of network N. Under such conditions, network N would function to maintain the temperature of the air blast in main 5 at the temperature required by the then This will be apparent from the following discussion of the structure of network N and its manner of operation.

The network N comprises a slidewire 91, whose position with respect to its contact or set that is, the driving point 90 corresponds with the existing temperature of blast air in main 5, and a slidewire 99 whose position with respect to its contact I corresponds with the setting of valve 1.

Thermocouple 9 produces a voltage responsive to changes in temperature of the blast air in main I. In the self-balancing potentiometer arrangement shown on the drawing, the voltage produced by thermocouple 9 is automatically balanced against the effective voltage of potentiometer slidewire I02. Upon change in air blast temperature, the galvanometer I02 deflects to effect, through mechanical relay mechanism I04, adjustment of the potentiometer slidewire I02 with respect to its contact I05 in such sense and to such extent that the voltage of thermocouple 9 is again balanced. The disc I06, upon which the potentiometer. slidewire I02 is mounted, is coupled, as by shaft I01, to the disc I00 upon which the slidewire 91 of network N is mounted, so that each different position of contact of set point 90 with respect to slidewire 91 corresponds with a different temperature of the air blast. The resistance of slidewire 01 is preferably, though not necessarily, concentrated in its central section of limited angular extent, for example, 60 degrees.

The current for the potentiometer slidewire I02, derived from any suitable source, such as a battery I09, is, from time to time, adjusted, as by variation of rheostat I09a to maintain it at standard value for which the slidewire I02 is calibrated. The usual standard cell and switching arrangement for effecting the standardization is not shown.

The mechanically relay mechanism I04, diagrammatically shown in Figure 4, may be of the type fully described and claimed in Squibb Patent No. 1,935,732. In brief, the pointer N0 of the galvanometer 103 is intermittently clamped against the stationary stop I I I by the clamping member II2 which is periodically rocked by a cam (not shown) on the shaft Ill continuously rotated at suitable speed, for example, thirty revolutions per minute. While the needle is so clamped, the feelers H4, H4 are released, by a cam (not shown) on shaft I I3, for movement toward each other by their biasing spring H5. If the pointer is clamped to right or left' of its neutral or central position, the lower end of one or the other of the feelers H4, H4 during its movement, engages the pin H0 extending from the driving clutch member 1' and swings it about its pivot II! in a sense and to extent corresponding with the sense and extent of deflection of the pointer I III from its neutral position. While so displaced from its neutral position, the driving clutch member III is moved, under the control of a cam (not shown) on the shaft Ill, into engagement with the driven clutch member II! attached to aforesaid shaft I01. Within the same revolution of shaft I I3, one or the other of the cams I20 engages driving clutch member I I1 and returns it to the neutral position shown in Figure 5 of the drawing, and since, during this time, the clutch members Ill and H0 are in engagement with each other, each of the slidewire discs I06 and I08 is moved in a direction and to an extent corresponding with the deflection of the galvanometer pointer.

For the purpose of explanation, it is assumed that when the temperature of the air blast is at a normal figure, the contact is an engagement with the midpoint of slidewire 91 and that slidewire I00 is moved in clockwise direction as the temperature falls, and in counterclockwise direction as the temperature rises. For brevity, that relative position of slidewire 01 and contact 98 which corresponds with the desired temperature is termed the normal control poin The end coils or resistances I2 I, I22 connected between the terminals of the slidewire 91 and the terminals I23", I24 of network N, determine the limits between which the ratio of the impedances of the two upper arms of the bridge is varied by adjustment of slidewire SI with respect to contact 88. The terminals I23, I24 of network N are connected to any suitable source of direct or alternating current. They may, for example, be connected to the secondary of a transformer (not shown) whose primary is energized from the alternating current line I25.

The lower pair of arms of the network N include the adjustable resistances I 26 and III connected between terminals of the slidewire 09. Q

and the aforesaid supply terminals I23, I 24 of the network.

The reversible motor M, of any suitable type, is suitably connected through shafting and gearing, genericallyillustrated by broken line I, to the mixing valve 1 and is controlled by a reversing switch comprising contacts I29, I30, and III, operated in response to unbalance of the network N by the relay R. of any suitable type, for example, a contact galvanometer. The motor M is energized from any suitable source of current connected to the terminals I32, I32 of its control circuit. The motor M is also mechanically coupled to the supporting disc Ill of the slidewire 00 to effect, upon unbalance of network N, adjustment of slidewire 90 with respect to its contact I00 in that sense required to restore the network N to balance.

It is characteristic of the control system thus far described that, upon change of air blast temperature, the network N is unbalanced by movement or slidewire 01" to an extent corresponding with the extent of the temperature change and is rebalanced by adjustment of slidewire 99' concurrently with actuation of the valve I by motor M. Accordingly, for every air blast temperature ther is a corresponding position or setting of valve 1.

When the relations between the extents of valve adjustment and the extents of adjustment of slidewires 91 and 99 are so selected that the system is stable, the temperature load characteristic of the system is a drooping one; that is, the control system tends to maintain the air blast temperature constant at a magnitude which becomes less and less with increase in volume of blast air to the furnace. Unless such relations producing a drooping characteristic are selected, the control system is unstable.

To maintain the air blast temperature substantially constant at the desired magnitude independent of fluctuations of the volume/min. Of the air blast, the network N is provided with another pair of arms comprising, or consisting oi, the resistors I34, I35 oi. material such as nickel having a substantial temperature coeiiicientoi resistance. The energization oi heater coil I33, which is in intimate heat transfer with resistor I34, is controlled by contacts; I31 133 oi relay RI responsive to unbalance oi a second or subsidiary bridge whose resistances I26 and I2'I' of the main bridge of network-N, is controlled by aforesaid contacts I313 I33, I40 of relay RI.

The magnitudes of resistance of resistors I34, I35, are so chosen that when the slidewire ll is in the position corresponding with the desired air blast temperature, the subsidiary bridge or network is in balance. Assuming, for example, the temperature of the air blast has fallen from the normal magnitude so as to reduce the impedance between terminal I 23 and contact 981 of the network and concurrently to increase the impedance between the contact 98' and terminal I24 of the network, th relay RI responds to the unbalance to energize the heater coil I39 gradually to increase the resistance of resistor I35 and so tend slowly to restore balance of the subsidiary network with contact 98 and slidewire 91' relatively displaced from the control point. Until such balance is restored, the contacts I38 and I of relay RI remain closed to eflect energization of motor MA for adjustment of resistances H26 and I21 in that sense unbalancing or tending to unbalance the main network N which causes, through relay Ri energization of motor M in that sense effecting further closing of valve I. This secondar control action, due to adjustment of resistors B26 and W5, is distinct from and in addition to the primary control action due to movement of slidewire ill in response to change in air blast temperature.

When the resistance of 335, because of the heat received from coil 039, has attained that value which effects rebalance of the subsidiary network, the relay Rt breaks the circuit to the heating coil 939 and to the motor MA, whereupon resistance 35 begins to cool with consequent decrease of its resistance.

iii the temperature of the air blast is rising toward normal magnitude thereof at the same rate that resistance Q35 is cooling, the auxiliary network remains in balance and there is no further adjustment of the droop-correcting resistors I28 and E21 by motor MA.

If the air blast temperature is rising toward normal magnitude thereof at a rate slower than the rate at which resistance 935 is cooling, the auxiliary network is again unbalanced in the same sense as before and relay Ri eiIects closure or contacts I 38 and I 40 again to energize the heater coil H9 and the motor MA to effect further adjustment Of the impedances I26 and I21; however, because the slidewire 91' has moved toward the control point, coil I39 is energized for a shorter time than before to effect rebalance. The cyclic heating and cooling of resistor I35 continues with increasingly shorter periods of energization of heater I39 and motor until the slidewire attains the control point.

It, on the contrary, the air blast temperature is increasing more rapidly than the rate at which resistor I35 is cooling, the auxiliary network is unbalanced in the opposite sense and relay RI responds to effect energization of heater coil I33 slowly to increase the eiiective magnitude of resistance I34 and also, by closure of its contacts I31 and I38, to efiect reverse adjustment of the resistances I26 and I21 by reverse operation of the motor MA. When the effective magnitude of resistance I34 has, because ofheat received from heater I38, attained that value suflicient to rebalance the subsidiary network, relay RI responds to deenergize heater I36 and motor MA whereupon resistance I34 begins to cool.

From the foregoing, it will be understood that when the air blast temperature increases above the desired magnitude, there is at once eflected a primary adjustment corresponding with or proportional to the deviation of slidewire 91 from the control point and that ther is subsequently, or more slowly, eiiected secondary adjustments of the valve 1 due to the cyclic balancing and unbalancing of the subsidiary network continuing until slidewire 91 returns to the control point.

Thus, the subsidiary network including the resistors I34, I35 and their associated heating coils I36, I39, prevents the main network N from remaining permanently in balance so long as the slidewire disc I08 is away from the position corresponding with the desired air blast temperature; so long as any deviation from said desired temperature exists, the subsidiary network is cyclically unbalanced to effect adjust ment of the impedances H26 and iii of the main network, in that sense providing for return of the air blast temperature to the desired magnitude and at a rate which progressively decreases as the desired temperature is aproached, thus to avoid overshootin or hunting.

The rheostats M2, M3 in series with the heater coils E39 and MS may be provided to provide for adjustment of the rate at which droop-corrective action is effected. increase in magnitude oi the series resistances 542, I53 requires the contacts of relay RI to be closed for a longer time to procure a given increase in temperature of the resistors i3 3 and and consequently motor Mi runs for a longer interval before a balance of the subsidiary network is effected. Rheostats M2, M3 may be mounted on the same shaft or on separate shafts, depending upon whether symmetrical or we symmetrical response is desired.

From the foregoing it is manifest that network N, acting with thermocouple 9 and mixer valve 5, will maintain the temperature of the blast air at the temperature required by the position of contact point @8. In accordance with the present invention, however, contact point 98 is not manually set, as was assumed above for ease of understanding that portion of the apparatus, but is in turn operated or set automatically by means responsive to the humidity of the unheated, indrawn blast air in such manner that the temperature of the air blast in main 5 is varied to compensate for variations in humidity, in order that the air blast may have a substantially constant heating effect upon the burden of the blast furnace.

The means for thus automatically setting contact 98 is shown in Figures 2A, 2B, and the upper part of Figure 3.. As shown in Figure l, unheated blast air in main 2 is drawn oil through pipe l2 in order to obtain a sample of such indrawn air. Such sample passes through throttle valve l3 and reducing valve [4. and then through pipe i2 (Figure 2A) into a means for constantly measuring the humidity of such sample.

Air passing through pipe I2 is led through air coils ll, said coils being immersed in constant temperature water in tank It. Constant temperature water is obtained from tank II, which is fed by tap water through pipe I I and valve 24, allowed to flow into tank H, in which are located steam coils i4, and thence through pipes 2! to tank It, from which it flows to the sewer through pipes 22. Exhaust steam from coils ll likewise flows to the sewer through pipe 24. Steam to coils I8 is fed through supply pipe 28 in amounts governed by the thermometer 26 located in tank It. Said thermometer, which may be of the fluid expansion type is connected by conduit 21 to the control device shown in Figure 28, to regulate valve 24 located between the steam supply 25 and pipe 22 in such manner that the water in tank it and consequently the air in coils i5 is brought to a predetermined constant temperature. The tanks I8 I and I1 and pipes 2| and 22, as well as the receptacle ll housing the wet bulb thermocouple, subsequently to be described, and the conduit 45 delivering constant temperature air to receptacle 46 from coils l5, are all carefully insulated to prevent radiation and conduction of heat therefrom, and to maintain the temperature of the air sample uniform and constant.

Control of valve 24 by thermometer 24 is effected by means of the control means shown in Figure 2B. The device shown for purposes of illustration is that disclosed in the patent to Harrison et al., No. 2,125,109, July 26, 1938, although it is obvious that other known control means may be substituted therefor. The control mechanism shown, as above explained, is for the purpose of adjusting valve 24 in response to thermometer 26to maintain a constant water temperature within tank I 6.

The fluid'pressure in chamber 26, which increases and decreases as the water temperature within tank It rises and falls, respectively, is

transmitted by conduit 21 to a pressure responsive element 28, shown as a Bourdon tube of helical form having its stationary end connected to the conduit 21', and having its movable end secured to an arm 29 which is pivoted to turn about an axis 30, clockwise or counterclockwise, as the pressure in the tube rises or falls, respectively.

Through suitable connections, the oscillations of thearm 29 give motion to the valve 3|, which as shown, forms part of a control pressure regulator or air'controller unit 32, and regulates the escape of air from, and thereby regulatesthe control air pressure in the regulator chamber 22. The latter receives air through a pipe 34. a coupling 25 and pipe 36 from air supply means, which as shown, comprises a chamber 31 receiving air under pressure through a supply pipe 28, and comprises means for maintaining a substantially constant pressure in the chamber, somewhat smaller than the pressure in the pipe 38, notwithstanding fluctuations in the last mentioned pressure. The flow through the pipe 34 into the pipe 26 is restricted as by means of restricted port 29, in the coupling 38 between the normally be the same as that in the chamber 22, and below that in the chamber 21.

The variable air pressure maintained in the chamber 32 is transmitted through a pipe 44 in free communication through the coupling II, with the pipe 26 to the pressure regulator diaphragm chamber 4i of a fluid pressure relay device having a main pressure chamber 42. The latter receives air under pressure through the passage 43 from a suitable source shown as the chamber 31, and means are provided for maintaining a pressure in the chamber 42 which varies with the pressure in the diaphragm chamber 4| and hence with the control pressure in the regulator chamber 22. The means shown for the purpose comprises a valve member 44 actuated by the flexible diaphragm 45 between chambers 42 and 4|, which throttles the air through inlet 42 more or less, and the outlet of air from chamber 42 through a vent 46, less or more as the pressure in the chamber 42 rises above or falls below the pressure in the chamber 4 I. The pressure in the chamber 42 is transmitted by a conduit 41 to the pressure chamber 48 of the valve 24, which is a fluid pressure valve adapted to admit either a heating or cooling fluid to the coils I8 through a supply pipe 23 as the control pressure in the regulator I2 rises and falls.

In the controller illustrated, movements of the arm 29 give movements to the valve 3| through a link 48, a rocking element 49, a floating lever 50, a link 5i connecting parts 49 and i0, and a bell crank lever 52 connected by pivot 52 to the lever 54, and carrying a pin 54 engaging the valve ii. The value of the control quantity which the apparatus tends to maintain may be manually adjusted, as conditions make desirable, by a rocking element 55 connected to the lever 40 by a link 51. The latter, in eifect, forms an adjustable fulcrum for the lever 50, and mechanism hereinafter described adjusts the lever 52 relative to the lever to modify the action of the latter on the valve ii, in response to variations in pressure conditions within the device 32, resulting from changes in the position or the valve 2|.

The rocking element 49 is mounted to oscillate about the axis of a shaft 58, and carries a pointer 59 which may indicate the value of the control quantity on a suitable scale in the instrument (not shown), and carries a pen point or stylus at its free end which is adapted to make a record on a chart disc (not shown). The chart is rotated by a constantly rotating chart shaft (not shown) and is provided with scale lines. As shown, the rocking element 49 includes an arm 60 to which the corresponding end of the link 29 is connected, and a second arm 6! to which the corresponding end of the link Si is connected.

The rocking element is mounted to oscillate about the axis of the shaft 58 and includes an index arm 62 which indicates on the same scale with which the pointer 59 cooperates the normal value of the control quantity. When the control quantity is at its normal value, the pointer II will be directly in front of, and will register on the scale with the index 42. As shown on the drawing, the position of the pointer 59 corresponds to an actual value of the control quantity greater than the normal value. The element ll includes an arm 43 to which the corresponding end of the link 51 is connected. In the valve operating means shown, the floating lever 54 is suspended from the arms SI and 42 by members pipel, so that the pressure in the pipe 84 may 76 II and I! which engage the lever 44 at spaced utilized in eliminating lost motion in eflecting an adjustment of the lever 58, through either of said links by angular adjustment of the corresponding elements 48 and 55. As previously explained, the element 49 is given angular adjustments in accordance with changes in the value of the control quantity acting through the parts 28, 28, and 48. The rocking element 55 may be manually adjusted about the axis of the shaft 58 to increase or decrease the normal value indicated by the index 82, and to simultaneously and correspondingly raise or lower the fulcrum for the lever 50 formed by the lower end of the link 51 by an adjusting element 84 having an arm 85 connected by a link 66 to the element 55. As shown, the adjusting element 54 is in the form of a rotatable element which may be normally held stationary by frictional means, and is provided with a. slot 61 for engagement by a screw driver by which the element 84 may be angularly adjusted to thereby angularly adjust the element 55, when adjustment in the normal value of the control quantity is desirable.

The device 32 as shown, comprises a cupshaped casing element detachably connected at its open end to a head 88. The former is formed with an aperture through which the air supply pipe 38 communicates with the space 33, and the latter, with an aperture in which is mounted the bleeder or vent nozzle 89 through which air escapes to the atmosphere from the chamber 33 at a rate depending on the adjustment of the valve member 3|. Preferably and as shown, the nozzle member 83 has an external globular end surface surrounding the outer end of the nozzle passage, and the valve member 3| is a piece of sheet metal extending transverse to the length of, and movable toward and away from the outer end of .the nozzle passage. As shown, the valve 3| is pivoted to turn about a shaft III which may be mounted on the head 58. A spring (not shown), coiled about shaft Ill, gives the valve 3| a slight bias toward its closed position, in which it engages the nozzle member 89 at the margin of the nozzle passage.

A resilient bellows element I| located within, but not filling the casing of the device 32, has one end secured to the end head 68, and has its other end closed so that an increase or decrease in the pressure within the space 33 between the casing of the device 32 and the bellows II, tends to contract and expand the latter. Within the bellows II and separated from the latter by an inter-bellows space I2, is a smaller bellows element I3 which has one end secured to the end head 58 at the margin of an opening I4 in the latter, through which theinterior'of the bellows I3 is in free communication with the atmosphere at all times. The inter-bellows space I2 communicates with the atmosphere through a restricted passage I5 formed in the end head 88. Advantageously, an adjustable obturator I6 is provided to regulate th flow capacity of the passage I5.

The movements of the free end II of the bellows I3 modify the operating effect of the lever 50 on the valve 3| through parts shown. which comprise a link 18 connecting the bellows head TI to a lever I9 mounted to oscillate about the shaft I0 and pivotally connected by a pivot shaft 88 to a lever 8|. The latter is connected by a pivot shaft 82 to the lever 52, and has an adjustable fulcrum formed by a pivot pin 83 carried by the lever 8| and slidingly received'in a diametral slot 84 formed in an angularly adjusted disc (not shown).

The link is which is axially disposed in the bellows I3, and may be rigidly secured to the end head II of the latter, is'preferably connected to the lever I8 for adjustment relative to the latter, in such manner as to vary the effective length of the link.

Clockwise movement of the lever 52 about its pivot 82 gives an opening movement to the flapper valve 3| through the action on the latter of the pin 54 carried by the lever 52. The pin 54 is advantageously connected to the lever 52 for movement toward and away from the pivot 82 to thereby vary the extent of valve movement produced by a given angular movement of the lever 52.

Each of the bellows elements II and I3 has longitudinal resilience and, consequently, a definite length when the pressures acting on its inner and outer walls are the same. When the external pressure on either bellows is less than, or exceeds the pressure within the bellows, the bellows will elongate or contract as required to make the differential of the effects of those pressures on the bellows equal to the opposing resilient bellows force resulting from the elongation or contraction of the bellows, that force, of itself, always tending to return the bellows to its normal or unstressed length. On an increase in the pressure within the Bourdon tube 28 and consequent movement of the link 5| upward, the lever 52 acts on the valve 3| to move the latter away from the nozzle 69 and thereby reduce the pressure in chamber 33. The pressure reduction in chamber 33 elongates the bellows II and thereby enlarges the inter-bellows space I2 and reduces the. pressure in the latter, owing to the relatively slow inflow of air permitted by the restricted passage I5. The reduction of the pressure in the space I2 elongates the bellows 13 and thereby moves the link I8 to the right. The movement thus given the lever I9 moves the parts 8|, 52 and 50 to the right and thereby permits of a return movement of the valve 3| toward the nozzle 89, partially neutralizing the effect of its initial movement away from the nozzle 69.

The extent of such return-movement of the valve 3| produced by a given movement of the link I8, depends upon the relation of the movement of the pivotal connection 82 to the movement of the pivotal connection 80, and that relation is dependent upon the inclination of the slot 84. With the slot 84 inclined generally as shown. a counterclockwise turning movement of the lever 1'9 about the shaft 18 will produce a clockwise movement of the lever 8| about the pivotal connection 88 accordingly, as the direction of the length of the slot 84 diverges counterclockwise, respectively, from the direction of the plane including the axes of the shaft 30 and 83. With the adjustment of the slot 84 such that the counterclockwise movement of the lever 19 causes the lever 8! to turn clockwise about the shaft 80, the angular movement of the pivot 82 about the shaft III will be less than the angular movement about the latter of the lever 19.

The enlargement of the inter-bellows space or chamber I2 reduces the pressure therein and thereby creates an inflow of air into I2 through the passage I5, which tends to restore the pressure in the chamber I2 to its normal equality with the pressure of the atmosphere. As the pressure in the chamber 12 builds up, the bellows position into which it has moved on the original change in position of the link Ii, provided the latter has not been moved in the direction of its length in the meantime as a result of a change in the Bourdon tube pressure. The converse of 12 sample from the cold blast main i2, will always present itself 'to the wet bulb thermocouple, or any other known humidity responsive element, at a constant temperature. Thus, any change in humidity of the air will evidence itself by changing the rate of evaporation at the wet bulb,

, or the like, with a consequent variance in the,

the actions just described occur on a decrease in ture of water through tank It to maintain the desired air temperature, the initial resultant decrease or increasein the water temperature will tend to produce a movement to its limit, of the valve controlling the media supply, in the direction to return the control temperature to its normal value, which will therefore be of such magnitude that the new media supply will be of higher or lower temperature, respectively, than that required to maintain the normal control temperature with the new temperature of incoming air. Such a control operation of itself, is essentially unstable, and must result in hunting. The control action described is modified, and the hunting tendencyeliminated or minimized by the initial movement of the link ll, which effects what is commonly called a follow-up adjustment. The latter can be made sufficient to in-- sure a control which is stable and not hunting. In such case, however, the follow-up adjustment, if not later neutralized, will result in the maintenance of a tube air temperature necessarily, and significantly lower with a vast incoming air to tube air temperature differential than with a lesser differential.

The control adjustment resulting from the second or return movement of the link ll, oc,-

curring as the pressure in the inter-bellows space I! approaches its normal atmospheric value, following a departure from that value, is a compensating adjustment which may be made to neutralize the effect of the follow-up adjustment slowly enough to substantially eliminate the effect of the follow-up adjustment on the control temperature when the tube air to incoming air temperature differential is steady, without giving rise'to an objectionable hunting tendency.

Returning to Figure 2A, constant temperature air, which is a sample of the air in main 2, after contacting the aforementionedjexpansion fluid thermometer 26 passes into contact with a wet bulb thermometer, or any other suitable device responsive to changes in humidity, whereby temperature of air is evidenced as function of its humidity. In the apparatus shown there is employed, for such purpose a wet bulb thermocouple ll, which is located in receptacle 8', and

which is constantly saturated by water through wick 94, contained in receptacle 9 I, which is supplied with water from inlet 92. Overflow water in receptacle 9| flows out through pipe 93. After flowing past wet bulb thermocouple 88 and dry bulb thermometer 81, which affords a constant check on the maintenance of uniform temperature of the air sample, the air is exhausted to atmosphere through opening I! and so, respectively.

It is now evident that the constant temperature air through air coils II, which is a '01)? humidity dependent temperature thereon. Thus; a differential of varying degree having one and only one variable has been established; that variable being the changes in humidity of air through air coils II. From this it can be seen that this diiferentlal can be utilized for controlling the aforementioned hot blastmixer valve for changes in humidity of incoming blast air to furnace.

As shown in Figure 3, leads and ll from wet bulb thermocouple II are led into network N, which is similar in all respects to network N. Corresponding elements in the two networks are similarly lettered and numbered, with the exception that in network N' the reference characters are primed. As will be apparent from the previous description of network N, the voltage produced by thermocouple II, which is responsive to the humidity of the indrawn, unheated blast air, is automatically balanced against the eflective voltage of potentiometer slidewire III. Upon change of humidity of such air, galvanometer Ill deflects to effect, through mechanical relay mechanism IN, adjustment of the potentiometer slidewire I02 with respect to its contact III in such sense and to such extent that the voltage of thermocouple O8 is again balanced. The disc I", upon which the potentiometer slidewire III is mounted, is coupled by shaft I I! to the disc I", upon which the slidewire 91 is mounted, so that each different position of contact of point it with respect to slidewire l1 corresponds with a different humidity of the unheated, indrawn blast air.

Reversible motor M, operated by relay R, will thus function to turn disc III, carrying slidewire 9!, to effect upon a balance of network N adjustment of slidewire 89 with respect to its contact ill! in that sense required to restore the network N to balance. Thus, upon change of humidity of the unheated, indrawn blast air, network N is unbalanced by movement of slidewire to an extent corresponding with the extent of the humidity change, and is rebalanced by adjustment of slidewire OI concurrently with actuation of the control or set point 8. of net work N' by motor M, acting through the driving means generically shown as "to. Accordingly, for every value of humidity of the unheated, indrawn blast air, there is a corresponding position or setting of control or, set point II of network N.

Driving means Illa may, if the positions of the parts allow it, be a shaft from motor M leading directly to a gear box driving an adjusting screw upon which is a, travelling nut connected to control or set point ll. It is usually more convenient, however, particularly when networks N and N' are in different cabinets, t employ an electrical mean to connect the shaft of motor M to the control point 88', such electrical means being of a type which retains a fixed known relationship between the speed of the transmitting and receiving means and a fixed relationship between the angular positions of the driving and driven shafts. The well-known selsyn generator and motor system may be employed as driving means me, the generator being connected to the shaft of motor M and the motor being connected to the adjusting screw for the control point 88' through a gear box, if necessary. Thus, there is a fixed relationship, depending upon the constants of driving means Ina, such as the gear reduction ratio and the pitch of the screw driving control point 98', between the amount that control point 98' is moved, and the amount which disc I33 rotates, as we have seen, depends upon the amount of change of the humidity of the unheated, indrawn blast air from a chosen base or standard value.

It has been found through experience that the ideal standard for the wet thermocouple setting is grains of moisture per cubic foot of indrawn, unheated blast air at 85 F. It has also been found that a rise or fall is temperature of the heated air blast entering the furnace of approximately 31 F. for each additional grain of moisture per cubic foot over or under such standard, respectively, will compensate for the increased or deceased chilling effect, respectivehr, of the added moisture in the unheated, indrawn blast air, and thus cause the heated blast to produce a. constant heating eflect upon the furnace burden. Therefore, the networks N and N and the various elements thereof are adjusted so that 5 grains of moisture per cubic foot at 85 F. is the standard, and the drive between disc I33 and control point 98' is made such that control point 98' is moved so that the temperature it demands is raised or lowered 31 F for each increment of movement of disc I33 corresponding to a rise or fall, respectively, of one grain of moisture per cubic foot, of unheated, indrawn blast air.

Since the various instruments employed, such as the means'for determining the humidity of the unheated, indrawn blast air and the means for determining the temperature of the heated blast air entering the furnace are provided with means continuously indicating and recording the phenomena which they detect and control, the operating may continuously observe how the control device is working, and any changes or adjustments of the apparatus to be made, such as are infrequently necessary, are readily apparent.

Although a particular embodiment of the invention has been shown and described for the purposes of illustration, it is obvious that numerous variations may be made by those skilled in the art within the scope of the invention. We therefore claim as novel the following.

We claim:

1. In apparatus for supplying hot ,blast air to a metallurgical blast furnace comprising a blast air intake pipe, means for heating a portion of the indrawn blast air, a hot blast supply pipe into which the heated air is discharged, means for mixing unheated air with the heated air to control the temperature of the blast, and a mixer valve for controlling the amount of cold air so mixed, the improvement which comprises an automatic control for the temperature of the blast, said control comprising means for measuring the temperature of the hot a r blast mixture, means responsive to the last named means to adjust the mixer valve to mainta n the temperature of such mixture at a predetermined set point, said means including an adjustable element the setting of which determines the temperature of the blast, means for continuously determining the moisture content per unit volume of the unheated air before being so mixed, said means comprising means for continuously isolating a sample portion of the unheated blast air, means for bringing such isolated portion to a reduced pressure and to a substantially constant temperature, a wet thermocouple, and means for causing the thus conditioned sample of the unheated blast air to impinge upon the wet thermocouple, and means automatically andcontinuously responsive to the wet thermocouple for raising and lowering the predetermined set point of the temperature control means as the moisture content of the unheated air before being so mixed rises and falls, respectively, said last named means comprising a balanceable electric circuit, a, first variable impedance in such circuit having an element movable as its impedance changes, a control circuit connected to the wet thermocouple, said control circuit including a self-balancing potentiometer ln'which the voltgae of the wet thermocouple is balanced against the effective voltage of thepotentiometer resistance, the potentiometer having an element movable as its effective voltage changes, means connecting the movable element of the potentiometer to the movable element of the first variable impedance to cause them to move in synchronism, a second variable impedance in a balanceable circuit, said second variable impedance having an element movable upon change of its impedance, the second variable impedance being similar to the first variable impedance and being symmetrically located in a balanceable circuit with respect thereto, means connecting the movable element of the second variable impedance to the temperature determining adjustable element of the temperature control means, means connected to the movable element of the second variable impedance to move it and the adjustable element of the temperature control means, and control means responsive to the movement of the movable element of the first variable impedance to cause the movement of the movable element of the second variable impedance in an amount corresponding to the movement of the movable element of the first variable impedance.

2. Apparatus for controlling the temperature of the hot blast for a metallurgical furnace which comprises means for heating the blast air, recording means for measuring the temperature of the hot blast air and means responsive to the temperature measuring means to maintain the temperature of the hot air blast at a predetermined set point, said temperature controlling means including an adjustable element the setting of which determines the temperature of the blast, means indicating the setting of the adjustable element, recording means for continuously determining the moisture content per unit volume of the unheated blast air, said last named means comprising means for continuously isolating a sample portion of the unheated blast air, means for bringing such isolated portion of the unheated blast air to a reduced pressure and to a substantially constant temperature, a wet thermocouple, and means for causing the thus condi- 18 said last named means comprising a balanceable electrical circuit, a first variable impedance in such circuit, having an element movable as its impedance changes, a control circuit connected to the wet thermocouple, said control circuit including a self-balancing potentiometer in which the voltage of the wet thermocouple is balanced against the effective voltage of the potentiometer resistance, the potentiometer having an element movable as its efiective voltage changes, means 10 m of m connecting said" movable element of the potentiometer to the movable element 01' the first variable impedance to cause then to move in synchronism, a second variable impedance in the balanceable circuit, said impedance having an element movable upon change of impedance and beingsimilartothefirstimpedanceandbeing symmetrically located in the balanceable circuit with respect thereto, means connecting the movable element of the second variable impedance to the set point or the temperature control means,

and control means responsive to movement of the s movable element of the first variable impedance to cause themovement or the movable element otthesecondvariablehnpedanceinanamount corresponding to the movement of the movable element of the first variable impedance.

TRUMAN H. mm. ONE '1. IORTBOU.

REFERENCES CITED Theiollowingreferencesareoirecordinthc UNITED STATES- PATINTB Number Name Date Re.20,083 Anderson Sept. 1; 1 1,187,740 Carrier Jan. 11, 1918 1,435,119 Hohl Nov. 7, 1832 1,458,288 Graham June 13, 1828 2,080,950 Otto la 18, 108'! 2,094,738 Dunham Oct}, 1m 2,171,308 Parks Sept. 8, 1880 2,300,537 Davis Nov. 8, 1843 145e,? King et a1. Jan. 11, 100

OTHER cm Clements, page 46 of "Blast I'urnace Practice,"

vol. III, published 1929 by Earnest Benn, Ltd, London, mland. 

